In general, a hard disk unit includes a medium having a magnetic recording film deposited thereon, a spindle motor for rotating the medium at a predetermined rotational speed, a magnetic head for writing and reading information data, a voice coil motor (VCM) for driving the magnetic head, a controller and the like. The recent drastic price competition among manufacturers imposes a demand for further cost reduction on voice coil motors as well.
Referring to FIG. 1, a voice coil motor 10 includes magnetic material yokes 1, 1 and magnets (permanent magnets) 2 adhesively disposed thereon as described in JP-A 2004-23969. It is noted that the magnet adhesively disposed on the upper yoke 1 is omitted in FIG. 1 for simplicity of illustration. The yokes 1, 1 are opposed and coupled by magnetic material posts 5 such that magnets 2 face each other to define a space therebetween. A moving coil 3 on an arm 4 is located in the space between magnets 2. The voice coil motor 10 has a function of positioning a magnetic head. After magnets 2 are disposed on yokes 1, they are magnetized so as to produce alternate magnetic poles in the order of N and S.
The shape of magnets is generally determined depending on the required performance of a voice coil motor. Referring to FIG. 2, a magnetic disk unit 20 is illustrated which includes a generally rectangular housing 22 for receiving a magnetic disk 21. The disk unit 20 further includes a pivot 23, an arm 24, a magnetic head actuator 25, and a suspension 26. The voice coil motor 10 is typically disposed at one corner of housing 22. To comply with this geometry, the yoke 1 is configured to (1) a triangular shape, typically right-angled triangular shape, including side portions 11, 11 opposed to the adjacent walls defining the one corner of the rectangular housing 22, an apex 12 defined by side portions 11, 11, and a bottom 13 opposed to apex 12, (2) a frusto-triangular shape in which the apex of a triangle is truncated or rounded, or (3) an arcuate, crescent or semicircular ring shape in which the bottom of a triangle at its center is recessed or cut-off in a semi-circular, triangular or apex-rounded triangular form. The magnet is also configured in conformity to the yoke 1, specifically to an arcuate, truncated arcuate or truncated inverse-V shape as illustrated in FIG. 3.
To form a magnet of an arcuate or similar shape, a mold of a corresponding shape must be prepared and worked such as by shaping, cutting, grinding or otherwise machining, which operation is expensive and less productive. In the event a magnet of an arcuate or similar shape is used, it is difficult to secure the magnet to the yoke at the registered position. A problem arises particularly in a compact magnetic disk unit because the accuracy of magnet registration has a substantial influence on the torque.